Permeability tuning loop antenna



y 1942- -v. D. LANDQN 2,290,825

PERMEABILITY TUNING LOOP ANTENNA Filed Jan. 27, 1940 3nnentor Patented July 21, 1942 GFFICE PERMEABILITY TUNING LOOP ANTENNA Vernon D. Landon, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 27, 1940, Serial No. 315,871

13 Claims. (01. 25020) This invention relates to tuning systems for radio receivers and the like, and more particularly to a method of and means for varying the permeability and tuning of a loop antenna circuit.

It has heretofore been proposed to utilize permeability tuning in the design of radio receivers. Such tuning is accomplished by mounting a magnetic core of minutely divided particles within each inductor which is'to be tuned over the frequency range of the receiver, and moving the cores by means of a tuning mechanism. One purpose of this arrangement is to eliminate the relatively expensive variable condenser. Another purpose is to provide a more compact receiver and, at the same time, to improve electrical performance of the various tuned circuits.

Permeability tuning ssytems present no unusual problems when the inductor coils ar properly designed to provide the desired range of frequency. In attempting to use a tuned loop antenna with a permeability-tuned receiver, however, it will be appreciated that the loop cannot be designed to function both as an ellicient pickup device and as aconcentrated inductance. The requirement for maximum voltage pickup dictates the provision of a loop antenna of large diameter mounted in an open space, but the necesssity for varying the resonant frequency Of the loop circuit over the broadcast spectrum dictates the provision of a small concentrated coil within the receiver chassis.

It is the principal object of this invention to provide means for effectively tuning the loop circuit of a receiver over the desired frequency range by means of a magnetic core Without sacrificing the essential pickup properties of the loop, and without resorting to complicated coupling systems or additional circuit elements.

Briefly, this object is accomplished by connecting an inductor in series with the loop antenna and varying the inductance of the former by means of a magnetic core.

It will be observed that a voltage of variable amplitude is impressed on the receiver input. Such voltage is a minimum at the low frequency end of the tuning range due to the decreased eifective height of the loop at low frequencies. It is a further object of this invention to provide an auxiliary loop antenna which is coupled to the receiver input, and which increases the input voltage at the low frequency end of th tuning range.

In the design of an economical receiver, it is highly desirable to utilize identical elements wherever possible. Consequently, it is advantageous to employ identical magnetic cores in each of the several tuned circuits of the receiver. Furthermore, when the various magnetic core elements are operatively connected to unicontrol means, it is also desirable to mov each magnetic core an equal distance when the control means is operated. Since the magnetic core which tunes th antenna'circuit varies only a portion of the total circuit inductance, the variation in the resonant frequency for a given movement of the core is somewhat less than that which would be obtained if the total circuit inductance were effected by the tuning means. Consequently, he other tuned circuits in the receiver, which do not include a fixed inductance corresponding to the loop inductance, must be compensated so that the required tracking between the various tuned circuits is achieved. It is a further object of this invention to provide such a compensating means.

Other objects of this invention, as well as abetter understanding of its nature and operation, will become apparent from the following specification when considered in connection with the accompanying drawing, in which Figure 1 is a circuit diagram of a permeabilitytuned loop input circuit; Figure 2 is a circuit diagram of, a similar loop input circuit which is provided with means for increasing the voltage pickup at the low frequency end of the tuning range; Figure 3 is a circuit diagram of a portion of a receiver illustrating a proposed method of compensating a tuned circuit so that it will track with the antenna circuit; and Figure 4 is a modification of the arrangement shown in Fig. 3. Similar reference numerals refer to similar elements throughout the several figures of the drawing.

Referring particularly to Fig, 1, a thermionic tube 5 represents the first radio frequency amplifier or the mixer tube of a conventional receiver. A loop antenna 1, preferably consisting of a single turn of substantial effective diameter, is connected between ground and one terminal of an inductor 9, the other terminal of which is coupled to the control grid of thermionic tube 5. Bias for the grid is obtained through a resistor I! from the A. V. C. circuit of the receiver, which is not illustrated in detail. The cathode of tube 5 is grounded. A small trimmer capacitor l3 is connected between the high potentialterminal of the variable inductor 9 and ground.

A core l5 of minutely divided magnetic particles is movably mounted within the inductor 9 and is connected to the receiver tuning means in any conventional manner. In order to vary the total inductance of the circuit including the loop I and inductor 9 over a range sufiicient to cover the broadcast band, it is necessary to keep the inductance of the loop antenna at a value which is less than approximately twent percent of th total circuit inductance when the magnetic core has been removed from coil 9. Consequently, the voltage pickup or the loop will be less than that of the conventional capacitytuned loop system. However, this disadvantage is overcome by increasing the loop diameter or effective area. It can be demonstrated that increasing the loop diameter by a factor of 1.7 will make the voltage pickup of the permeability-tuned system equivalent to that of the capacity-tuned loop system at the high frequency end of the band of operation.

I have found that it is highly desirable to connect the loop antenna in the low or ground potential end of the input circuit, since this tends to greatly reduce the capacitive pickup of the loop in that its distributed capacity to ground is essentially uniform. Signal-representing volt.- ages are induced in the loop antenna by reason of its response to signals arriving from a given direction which induce circulating currents in the loop. By decreasing the capacitive pickup, sometimes called the vertical antenna eifect, the directivity of the loop is increased and, in addition, the signal-to-noise ratio is increased.

While I have indicated above that a singleturn loop is preferable, the available voltage may be increased by increasing the number of turns in the loop antenna provided its inductance does not increase at the same time to such a value that the circuit cannot be tuned over the required range. For a given number of turns, the loop inductance is a minimum when the turns are most widely spaced. Consequently, in a multiturn loop, the turns should be spaced as far apart as possible to; minimize the inductance. Flor a given inductance, a single-turn loop of large diameter has more pickup than a loop of smaller diameter having a plurality of turns, and. is therefore preferred.

Referring now to Fig. 2, the low inductance loop antenna l is connected in series with the permeability-tuned inductor 9, as before. In addition, an auxiliary loop antenna ll has been provided. This loop antenna may be coaxially mounted with the single-turn loop antenna 7, as

illustrated, or it may be mounted in a different location or at a diiferent angle. The auxiliary loop H is connected ,to a coupling coil is which is coupled to the permeability-tuned inductor 9. As a result, when the voltage applied to the grid of the amplifier tube 5 from the single-turn loop I tends to decrease at the low frequency end of the band, a signal voltage derived from the auxiliary loop fl is coupled into the input circuit tending to maintain the receiver sensitivity at a substantially constant level.

When the tuning inductor 9 and the coupling inductor i9 are identical and 100 percent coupled, the shunt loop I! is effectively in parallel with the tuning inductor 9, and tends to decrease its effective inductance. This unity ratio of inductances is obviously not preferable. When the tuning and coupling inductors 9 and I!) are not identical, that is, when they consist of different numbers of turns, T2 and T1, respectively, but are still approximately 100 percent coupled, then the shunt loop ll may still be considered as being in parallel with the tuning inductor 9 provided the inductance of the loop is in creased by the ratio i In this case, the pickup voltage of the shunt loop is increased by the ratio 5 1 Under the above conditions, if the apparent inductance of the auxiliary loop (inductance of loop multiplied by is approximately ten times the maximum inductance of the inductor 9, the effective tuning range of the inductor is decreased approximately 5 per cent. When the auxiliary loop is used, the inductance of the single-turn loop l is preferably not greater than 10 percent of the total circuit inductance at the high frequency end of the band. The effect of thesingle-turn loop 7, in series with the variable inductor 9, is to reduce the effective tuning range approximately 5 percent. The two loop antennas, therefore, produce a net reduction of the tuning range of approximately ten percent of that which could be obtained with the variable inductor alone. This reduction in the tuning range, however, is not sumcient to cause serious concern.

The single-turn or series loop 1 is chiefly effective at the high frequency end of the operating band, since it then represents approximately ten percent of the total circuit inductance. At low frequencies, the inductance of the inductor 9 increases so that the loop inductance then becomes a smaller percentage of the total. The auxiliary loop antenna H is chiefly effective at the low frequenc end of the operating band, since its apparent inductance then represents but ten times the inductance of the inductor 9 across which it is effectively connected. At high frequencies, the variable inductor 9 has a smaller effective inductance, since the iron core has been withdrawn, and the effective ratio of the shunt loop inductance to that of the inductor is much larger than 10 to 1.

Referring now to Fig. 3, a circuit embodying the series loop circuit and a second permeabilitytuned circuit is shown. The latter circuit represents a successive stage of radio frequency amplification, but it is to be understood that the same principle is likewise applicable to an oscillator circuit.

The anode 5 of the first amplifier 5 is connected to a primary winding 2!. The secondary circuit includes a variable inductor 23 and a correcting coil 24, the latter being coupled to the primary winding 2| as a secondary coil. The inductance of the variable inductor 23 is varied by means of a magnetic core 25 which is operatively connected to the first magnetic core 15, both of which are driven by a tuning mechanism 21. The tuning mechanism illustrated is intended to be merely an example of one of many known systems for varying the position of the magnetic cores. For example, a tuning knob 29 which is connected to a cam 3! controls the position of the magnetic core i5 through the medium of a cam follower shaft 35.

It is necessary for the resonant frequency of both the antenna input circuit and the amplifier secondary circuit to be the same at all positions of the tuning control knob 29. The effective change of resonant frequency of the antenna circuit is reduced by reason of the effects of the two loop antennas, as pointed out above. A similar reduction of the effective tuning range of inductor 23 is accomplished, in accordance with this invention, by means of the correcting or secondary coil 24. The inductance of the correcting coil is chosen so that it represents a constant portion of the total secondary circuit inductance equivalent to that introduced into the antenna circuit by the loop antenna or antennas, as the case may be.

The correcting coil 24 is preferably mounted so that it is closely coupled to the primary winding 2!, and may be shielded from the variable inductor 23 by means of a shield 26. The inductor 23 is also shielded from, or loosely coupled to, the primary winding 2|, so that the coupling of the transformer is determined largely by the correcting coil and is not aifected by the movement of the magnetic core 25. Alternatively, the length or diameter of the inductor 23 may be increased to a point which provides the desired compensation, in'which case the inductor 2-3 must be directly coupled to the primary 2i as a secondary coil. The latter method, however, is not preferred, since it requires the use of nonuniform coils, and has the additional disadvantage of introducing certain variations in the ef fective resonant frequency of the circuit due to end effect.

Fig. 4 illustrates the application of the series correcting coil to an impedance coupled amplifier. In this instance the plate circuit of the amplifier includes the variable inductor 3L and correcting inductor 31, serially connected between the plate and the positive voltage supply source, but not mutually coupled. The inductor 33 is tun d by a magnetic core 353. A trimmer capacitor 4! is provided for aligning the high frequency end of the tuning range, as in the previous circuits. The plate 6 of amplifier 5 is coupled to the grid 53 of the mixer tube 45 by means of a conventional capacitor 4Tresistor 49 network.

I have thus described several circuit ments for providing permeability tuning for a loop antenna having satisfactory signal strength and an improved signal-to-noise ratio. t is to be understood, however, that various modifica tions will occur to those skilled in the art which are within the scope of this invention. The specific arrangements shown are to be taken merely by way of illustration and not as limitations.

I claim as my invention:

1. In a radio receiver having a thermionic tube, an input circuit connected to said tube and including a loop antenna and a variable inductor serially connected between an input electrode of said tube and a point of ground potential for said receiver, the inductance of said loop being of. the order of ten percent of the minimum inductance of said inductor, a magnetic core associated with said inductor for tuning said input circuit over a range of frequencies, an auxiliaryloop antenna effectively coupled in shunt circuit relation to said inductor, the apparent shunt inductance of said auxiliar loop being of the order of ten times the maximum inductance of said inductor.

2. A radio receiver having a plurality of tunable circuits, one of said tunable circuits comprising a serially-connected loop antenna and a first inductor; means associated with said first inductor for varying its inductance over a predetermined range; another of said tunable circuits comprising second and third serially-connected inductors whose inductances correspond respectively to the inductances of said loop and said first inductor; and means associated with said third inductor for varying its inductance over a predetermined range.

8. A radio receiver having a plurality of tunable circuits, one of said tunable circuits comprising a serially-connected loop antenna and a first inductor, first magnetic core means associated with said first inductor for varying its inductance over a predetermined range; another of said tunable circuits comprising second and third serially-connected inductors whose inductances are equal respectively to the inductances of said loop and said first inductor; and second magnetic core means operatively connected to said first magnetic core means for simultaneously varying the inductance of said third inductor over said predetermined range.

4. A radio receiver having a plurality of tunable circuits, one of said tunable circuits comprising a serially-connected loop antenna and a first inductor; first magnetic core means associated with said first inductor for varying its inductance over a predetermined range; another of said tunable circuits comprising second and third serialiy-connected inductors whose inductances correspond respectively to the inductances of said loop and said first inductor; and means including a second magnetic core operatively connected to said first magnetic core for simultaneously varying the inductance of said third inductor over a predetermined range of frequencies.

5. In a radio receiver; a loop antenna and an inductor serially connected to the input circuit of said receiver; means for varying the inductance of said inductor to tune said antenna and inductor over a range of frequencies; and an auxiliary loop antenna magnetically coupled to said inductor and having an apparent inductance effectively in shunt with said inductor, the ratio of said apparent inductance to the inductance of said inductor becoming less as the inductance of said inductor is increased near the low frequency end of said range of frequencies for increasing the response of said receiver at the low frequency end of said range of frequencies.

6. In a radio receiver, a loop antenna and an inductor serially connected between the input terminals of said receiver, th inductance of said l op being small with respect to the minimum inductance of said inductor; a magnetic core associated with said inductor for varying the resonant frequency of said serially connected loop and inductor over a range of frequencies; and an auxiliary loop Whose inductance is large with respect to that of said inductor, magnetically coupled to said inductor, said loop having an apparent inductance in shunt with said inductor, the ratio of said apparent inductance to the inductance of inductor decreasing near the low frequency of said for increasing the response of said receiver at low frequency end of said range.

'2. In a radio receiver, a loop antenna and an inductor SeIl llv connected between the input .1 of said receiver; a magnetic core associated with said inductor for varying the resonant frequency of said serially connected loop and inductor over a range of frequencies, and an auxiliary loop whose inductance is large with respect to that of said inductor coupled to said inductor in such manner that the ratio of the apparent loop inductance in shunt with said inductor to the inductance of said inductor decreases near the low frequency end of said range for increasing the response of said receiver at said low frequency end of said range.

8. In a radio receiver having a thermionic tube, an input circuit for said tube including an in.- ductor and a loop antenna connected between the grid electrode of said tube and a point of ground potential for said receiver, the inductance of said loop being of the order of percent of the minimum inductance of said inductor; a magnetic core associated with said inductor for varying its inductance to thereby tune said input circuit over a range of frequencies, the voltage applied to said receiver by said 100p circuit decreasing at the low frequency end of said range; and an auxiliary loop antenna coupled to said input circuit, said auxiliary loop antenna having an apparent inductance in shunt with said inductor, the ratio of said apparent inductance to the inductance of said inductor decreasing near said low frequency end of said band to thereby apply maximum signal to said receiver at said low frequency end to compensate for the de crease in signal from said loop antenna.

9. In a radio receiving system, a receiver having a thermionic tube, an input circuit for said tube including an inductor and a first loop antenna connected in series circuit relation between a control electrode of said tube and a point of ground potential for said receiver, the inductance of said loop being of the order of ten percent of the minimum inductance of said inductor, a magnetic core associated with said inductor for varying its inductance to thereby tune said input circuit over a range of frequencies, the voltage applied to said receiver by said series loop circuit decreasing at the low frequency end of said range, and a second loop antenna coupled in shunt circuit relation to said inductor, said second loop antenna having an apparent shunt inductance substantially greater than ten times the minimum inductance of said inductor and only about ten times the maximum inductance of said inductor, to thereby apply increased signal voltage to said receiver at said low frequency end to compensate for the decrease in signal from said first loop antenna circuit.

10. In a radio receiving system, a receiver, a variable tuning inductor connected in the input circuit of said receiver for tuning said circuit throughout a range of signal frequencies, a relatively small inductance loop antenna connected in series circuit relation with said inductor in said input circuit and having an inductance that is small relative to the minimum inductance of said inductor, and a relatively large inductance loop connected effectively in shunt circuit relation with said inductor and having an apparent inductance that is larger than the maximum inductance of said inductor, said series circuit being relatively more effective in impressing signal voltage of the higher frequency end of said range on said receiver, and said shunt circuit being relatively more eiiective in impressing low frequency signal voltage of the lower frequency end of said range on said receiver.

11. In combination in the input circuit of a radio receiver or the like, a relatively small inductance loop antenna for picking up signals, a variable tuning inductor connected in series therewith in said input circuit for tuning same throughout a broadcast frequency range, the inductance of said antenna being a small percentage of the minimum effective series inductance in said input circuit, the percentage becoming smaller and the signal voltage available from said antenna decreasing as said inductor is tuned toward the lower frequency end of said range, and a relativel large inductance loop antenna for picking up signals connected in said input circuit, the inductance of said large inductance p bearing a large eifective ratio to the minimum inductance of said tuning inductor, said ratio becoming smaller and the available signal voltage from said large inductance loop antenna increasing as said inductor is tuned toward said lower frequency end.

12. In combination in the input circuit of a radio receiver or the like, a relatively small inductance loop antenna and a relatively large inductance loop antenna connected in the input circuit of a radio receiver, and a variable tuning inductor connected in series relation with said small inductance loop and in shunt relation with said large inductance loop in said input circuit for tuning said loop antennas throughout a frequency range, said small inductance loop having substantially less inductance than the minimum of said inductor and being characterized by the impression on said receiver of a decreasing signal voltage as said inductor is tuned toward the lower frequency end of said range, said large inductance loop offering a large effective ratio of inductance to the minimum of said inductor, said ratio becoming smaller and the available signal voltage from said large inductance loop increasing as said inductor is tuned toward the lower frequency end of said range.

13. In combination in the input circuit of a radio receiver or the like, a relatively small inductance loop antenna and a relatively large inductance loop antenna, said circuit including inductance in series with said small loop antenna and in shunt with said large loop antenna, and core tuning means for varying said inductance to tune said circuit over a frequency range, the inductance of said small inductance loop being a relatively small percentage of the minimum effective series inductance in said input circuit, the percentage becoming smaller and the signal voltage available from said small inductance loop decreasing as said means tunes said circuit toward the lower frequency end of said range, the inductance of said large inductance loop antenna bearing a relatively large effective ratio to the minimum of said shunt inductance, the ratio becoming smaller and the signal voltage available from said large inductance loop increasing as said means tunes said circuit toward said lower frequency end, thereby compensating said decreasing signal voltage from said small inductance loop.

VERNON D. LANDON. 

